The present invention relates to a drive apparatus for independently changing the phase of the moon for use in a planetarium, more particularly to a drive apparatus for changing the phase of the moon selectively as related to annual motion or independently thereof.
Generally to continuously project the changing lunar phase by a planetarium, the image of the moon projected onto the dome is moved from west to east or from east to west through an angle required for the cycle of the lunar phases. In other words, the moon image is brought into annual motion. However, since the dome of planetarium represents only the upper half of the celestial sphere for observation, it is impossible to demonstrate the changes in the moon phase during the travel of the moon beneath the horizon of the celestial sphere.
In order to represent one complete cycle of the lunar phases on the dome of a limited area, stars other than the moon, namely the sun, Saturn, Mercury, Venus, Mars, Jupiter and the like planets and fixed stars to be projected by the star globe must be moved in the reverse direction for diurnal motion at the same velocity as the angular velocity of the moon as it travels on the celestial sphere so as to offset the increase of right ascension of the moon itself with the reverse diurnal motion. Otherwise it would be impossible to project a continuously waxing and waning moon image onto the dome.
It is seen from FIG. 1 that when the planetarium projector is operated for annual motion, the position of moon image continuously changes with respect to right ascension along the moon's path, since the moon travels along its orbit in the direction of the arrow A as time elapses while waxing and waning. Furthermore when the intersection of the moon's orbit and the ecliptic coincides with or is positioned close to the vernal equinox as illustrated in FIG. 1, the moon's path forms an angle of inclination of 23.degree.27' + 5.degree.9' = 28.degree.36' with the celestial equator. Accordingly, even if the increase of right ascension of the moon is offset by the reverse diurnal motion in an attempt to project the image of the moon at a fixed position, the annual motion of the moon takes place reciprocally in the direction of declination in the range of from 28.degree.36' north to 28.degree.36' south, namely over the entire range of 57.degree.12' in declination. Thus even if the increase in the right ascension involved in the annual motion of the moon in the northern hemisphere can be cancelled by the reverse diurnal motion, it is also necessary to cause a latitude changing motion as related to the reverse diurnal motion. This makes the planetarium complex in construction and cumbersome to operate and changes the relative positions of the sun, Saturn, Mercury, Venus, Mars, Jupiter and like planets to be projected by the projectors which are adapted to be operated commonly by the drive shaft for producing the annual motion of the moon and for changing the lunar phase. After operation, furthermore, there arises the necessity to return the moon, sun and five great planets to the original positions and to restore the original diurnal motion.
Accordingly it is difficult even for a skillful operator to operate the planetarium in the above-mentioned manner as smoothly as is desired for educational and general purposes.
In view of the foregoing problems, a planetarium has been provided which is equipped with a moon projector operatively related to an annual motion mechanism and another projector for projecting moon phases independently of the annual motion mechanism so as to simplify the construction and operation. With such planetarium changes in the moon phase are reproduced by the former projector, and when the moon phases are to be projected by the independent projector, annual motion is stopped, simultaneously turning on the latter projector and bringing the former projector out of operation. Alternatively, the latter projector is turned on for projection independently of the moon phase projected by the former projector.
Thus the known planetarium requires two moon projectors and drive means therefor. Moreover when the projection of lunar phases in operative relation to the moon's annual motion is switched to the projection of moon phases independent of the annual motion, the lunar phase to be projected by the independent projector upon switching must be brought into conformity, solely by the knack of the operator, with the lunar phase previously projected by the annual motion-related moon projector just before it is turned off. Such operation is very troublesome, possibly entailing a discrepancy in the lunar phase. When the independent moon phase projector is turned on for projection independently of the moon phase projected by the annual motion-associated moon projector, no consideration is given to ensure coincidence between the two moon phases, with the result the discrepant lunar phases give an unnatural impression to the observer.